原式=2sin(α+π/6)cos(π/6)=-(4/5)*√3
=sin(α+π/6)=-4/5
所以
cos(α+π/6)=3/5
cosa=cos(α+π/6-π/6)=cos(α+π/6）cos(π/6)+sin(α+π/6）sin(π/6)=
(3√3-4)/10